1. Field of the Invention
This invention relates to reflective light valve systems and more particularly concerns decreasing DC induced transient optical effects in a liquid crystal light valve.
2. Description of Related Art
The liquid crystal light valve (LCLV) is a thin film, multi-layer structure comprising a liquid crystal layer, a dielectric mirror layer, a light blocking layer and a photosensitive layer, all sandwiched between two transparent electrodes. In a typical reflective LCLV projection system, a high intensity polarized projection beam is directed through the liquid crystal layer to the dielectric mirror. An input image of low intensity light, such as that generated by a cathode ray tube, is applied to the photosensitive layer, thereby switching the electric field across the electrodes from the photosensitive layer onto the liquid crystal layer so as to selectively activate the liquid crystal according to the intensity of input light received at different areas of the photosensitive layer. Linearly polarized projection light from a high power light source, such as a Xenon lamp, passes through the liquid crystal layer and is reflected from the dielectric mirror. The light reflected from the mirror is polarization modulated by the liquid crystal in accordance with the light information incident on the photo responsive layer. Therefore, if a complex distribution of light, for example a high resolution input image from a cathode ray tube, is focused onto the photosensitive layer surface, the liquid crystal light valve converts the low intensity input image into a replica image which can be reflected for projection with magnification to produce a high brightness image on a viewing screen. Projection systems of this type are described in several U.S. patents, including U.S. Pat. Nos. 4,650,286 to Koda, et al for Liquid Crystal Light Valve Color Projector; 4,343,535 to Bleha, Jr. for Liquid Crystal Light Valve; 4,127,322 to Jacobson, et al for High Brightness Full Color Image Light Valve Projection system; and 4,191,456 to Hong, et al for Optical Block For High Brightness Full Color Video Projection System.
The photo sensitive layer of the liquid crystal light valve, whether a silicon photo sensor or a cadmium sulphide photo sensor, exhibits rectifying and photo voltaic properties that cause DC offsets within the liquid crystal light valve. These DC offsets, which transiently vary as they adjust to an equilibrium condition, result in spurious transient optical output intensities ("overshoot") that degrade operation of the light valve. This overshoot effect is particularly noticeable when input light is switched between off and on. As output light intensity increases when input light is switched on, output intensity momentarily and undesirably increases to a level beyond the steady state because of the DC induced transient overshoot. Applicants are not aware of any prior methods for handling this problem.
Accordingly, it is an object of the present invention to avoid or minimize effects of DC induced transient optical effects in a liquid crystal light valve.